This invention relates to the art of monitoring the hydrostatic pressure of drilling fluid in the mud separator to adequately suppress the differential changes of the formation pressure encountered during oil and gas well drilling operations. More particularly, it relates to monitoring parameters associated with the mud separator correlatable to the minimal hydrostatic pressures of the drilling fluid against the formation pressure necessary to avoid blowout hazards.
The invention further relates to monitoring parameters in air drilling operations to determine the volume of injected gases and formation gases being circulated and avoid hazards of formation gases entering the aqueous liquid pit.
Accepted drilling operations utilize drilling fluid or "mud" to provide hydrostatic pressure against the formation pressure to prevent the formation pressure from exceeding the hydrostatic pressure of the mud and consequently causing a blow out of the well. In the drilling procedure for oil and gas wells it is common to circulate mud through the hollow drill stem, beyond the drill bit and return it between the drill stem and bore hole or casing. Upon return of the mud to the surface it is transferred to a mud settling pit to settle out the solid cuttings and the mud is recycled. Generally, the mud is processed through a mud separator before going to the mud settling pit. The function of the mud separator is to separate entrained gas from the mud and the solid cuttings and prevent gaseous hydrocarbons from entering the mud pit which could create a disastrous hazard if hydrocarbons got into the mud pit and were ignited. It should be noted that quantities of oil entrained in the mud are not separated at the mud separator. In most cases removal of oil from the mud is a separate operation.
In air drilling operations the mud separator is used to separate aqueous liquids, which are used in air drilling operations to provide lubrication of the drill bit, from injected gases and formation gases and further permits disposal of the formation solid cuttings.
Not only is well blowout a hazard in oil and gas drilling operations, but also blow over into the mud pit from the mud separator if the gas pressure in the separator exceeds the hydrostatic pressure of the mud leg in the mud separator. This could occur if preventative or corrective action is not taken timely.
Some present drilling operations are conducted where the hydrostatic pressure of the mud is less than the formation pressure, which is frequently referred to as under balanced drilling. Such operations increase the bit penetration rate, with a generally longer bit life thus decreasing the cost of drilling the well and decreases the risk of fracturing a low pressure formation by forcing drilling mud into the formation. Thus, it is certainly desirable in drilling operations to conduct under balanced drilling.
Often in drilling oil and gas wells extremely high pressure gas pockets will be encountered with the potential of the well blowing out. However, frequently the high pressure pockets are of such a low porosity that even though high pressure exists in the pores of the strata not enough of a volume of the high pressure gas gets into the well bore to cause an immediate concern. Such encounters of high pressure pockets are reflected in the mud returned to the surface as entrained gas which lessens the density of the drilling fluid and consequently the drilling fluid exerts less hydrostatic pressure. Further, when liquid hydrocarbons are encountered the hydrocarbons further reduce the density of the drilling fluid. When such drilling fluids reach the mud separator the density of the drilling mud may very significantly from the initial density thus the density of the drilling mud in the mud separator may have a density of 95% of the original density.
U.S. Pat. No. 3,365,009 issued to Burnham discloses a method having flow parameter regulating means for controlling the flow rate and pressure of drilling fluid emanating from a well and gas separating means for liberating gas entrained in the fluid prior to recirculating. The regulating apparatus includes a bladder valve with an actuating chamber which receives compressed gas for flexing the resilient bladder to achieve the desired size of the control passageway through the bladder, thus the pressure ratio across the bladder valve may be varied by varying the control passageway.
U.S. Pat. No. 5,010,966 issued to Stokley teaches a method of receiving a return of drilling fluid from a well being drilled, in which the hydrostatic pressure of the drilling fluid is less than the formation pressure, and controlling the flow and pressure of the return, separating oil and gas from the drilling fluid at the surface, and then returning the drilling fluid to the well and separating the oil and gas phases for further disposition.
U.S. Pat. No. 2,314,169 to Wilson discloses a method for detecting gas in well drilling fluids and in particular a method and apparatus for separating and detecting the minute amounts of gas in the drilling fluid during drilling for determining the location of the strata source of the gas.
U.S. Pat. No. 3,213,939 issued to Records discloses a method and apparatus which involves maintaining a desired back pressure on the drilling fluid or mud by means of a controlled gas pressure, which pressure together with the column of drilling fluid assure that a well blow out is prevented.
U.S. Pat. No. 3,498,393 issued to West discloses a method of blow out protection wherein the mud returned to the surface is introduced into a separator and gases retained in the mud are separated from the mud. The gas is then passed through appropriate size lines wherein instruments are located which measure the volume of gas flow by such measurements the operator is appraised of increases and decreases of gas flow rates in sufficient time to take appropriate action as required. The system is designed for drilling operations in which the least possible hydrostatic head is maintained by the drilling fluid.
Assuming constant permeability of the gas strata, from which the gas in the return mud emanates, the flow rate of the gas from the mud separator is comparable to the pressure in the bottom of the well. Comparing the flow rate measurements of the gas from the separator with measurements taken earlier, the rate of change in the flow rate of the gas from the mud separator may be determined. These measurements thus allow the driller to predict what is happening down hole at any given time and then adjust the hydrostatic head by increasing or decreasing density of the mud.